JP2023171464A - Measurement device and measurement method - Google Patents

Abstract

To easily change the shift amount and/or the shift direction for shifting the frequency of a frequency shift feedback laser.SOLUTION: Disclosed is a laser device for outputting the frequency-modulated laser light in a plurality of modes, which includes: an amplification medium which amplifies the input light; a laser resonator having an optical SSB modulator for performing frequency-shifting on the frequency of the light amplified by the amplification medium; and a control part for controlling the optical SSB modulator to perform the frequency-shift of the frequency of light to be inputted to the optical SSB modulator. The laser device, a measurement device and a measurement method are provided.SELECTED DRAWING: Figure 1

Description

The present invention relates to a laser device, a measuring device, and a measuring method.

2. Description of the Related Art A frequency shifted feedback laser (FSFL) is known, which includes a frequency shifter in a resonator and outputs a plurality of longitudinal mode lasers whose oscillation frequency changes linearly over time. Further, an optical rangefinder using such a frequency-shifted feedback laser is known (see, for example, Patent Document 1 and Non-Patent Document 1). Further, as a frequency shifter, an optical SSB (Single Side Band) modulator is known (see, for example, Patent Documents 2 and 3).

Patent No. 3583906 specification Patent No. 3867148 specification Patent No. 4524482 specification

Takefumi Hara, “Distance sensing using FSF laser and its applications,” Opto News, Vol. 7, No. 3, 2012, pp. 25-31

As such a frequency-shifted feedback laser, a configuration in which a frequency shifter using an acousto-optic element is provided in a resonator is known. An acousto-optic element is an element that outputs diffracted light at a frequency shifted by the frequency of the ultrasonic signal from the frequency of the input light when light is input into a region within the element where an ultrasonic signal is propagating. Such an acousto-optic element cannot easily change the frequency to be shifted and the sign or negative of the frequency. Therefore, in a frequency-shifted feedback laser using such an acousto-optic element, the amount and direction of frequency shift cannot be easily changed.

The present invention has been made in view of these points, and it is an object of the present invention to enable the shift amount and/or shift direction for shifting the frequency of a frequency shift feedback laser to be easily changed.

In a first aspect of the present invention, there is provided a laser device that outputs frequency modulated laser light in a plurality of modes, which includes an amplification medium that amplifies input light, and a frequency shift of the frequency of the light amplified by the amplification medium. The present invention provides a laser device comprising: a laser resonator having an optical SSB modulator; and a control unit that controls the optical SSB modulator to shift the frequency of light input to the optical SSB modulator.

The control unit may set a frequency shift amount of the optical SSB modulator.

The optical SSB modulator includes a substrate, a main Mach-Zehnder waveguide provided on the substrate and having a first arm waveguide and a second arm waveguide, and a first main Mach-Zehnder waveguide provided in the first arm waveguide. It has a sub-Mach-Zehnder waveguide and a second sub-Mach-Zehnder waveguide provided in the second arm waveguide, and the control unit controls the main Mach-Zehnder waveguide, the first sub-Mach-Zehnder waveguide, and the second sub-Mach-Zehnder waveguide. The frequency shift amount may be set by supplying a DC voltage and an RF signal of predetermined values to electrodes provided on the substrate corresponding to the two-sub Mach-Zehnder waveguide, and changing the frequency.

The control unit may set a frequency shift direction of the optical SSB modulator.

The optical SSB modulator includes a substrate, a main Mach-Zehnder waveguide provided on the substrate and having a first arm waveguide and a second arm waveguide, and a first main Mach-Zehnder waveguide provided in the first arm waveguide. It has a sub-Mach-Zehnder waveguide and a second sub-Mach-Zehnder waveguide provided in the second arm waveguide, and the control unit controls the main Mach-Zehnder waveguide, the first sub-Mach-Zehnder waveguide, and the second sub-Mach-Zehnder waveguide. The frequency shift direction may be switched by supplying a DC voltage of a predetermined value to electrodes provided on the substrate corresponding to the two sub-Mach-Zehnder waveguides.

The optical SSB modulator includes a substrate, a main Mach-Zehnder waveguide provided on the substrate and having a first arm waveguide and a second arm waveguide, and a first main Mach-Zehnder waveguide provided in the first arm waveguide. It has a sub-Mach-Zehnder waveguide and a second sub-Mach-Zehnder waveguide provided in the second arm waveguide, and the control unit controls the main Mach-Zehnder waveguide, the first sub-Mach-Zehnder waveguide, and the second sub-Mach-Zehnder waveguide. The frequency shift direction may be switched by inverting the phase of the RF signal supplied to the electrodes provided on the substrate corresponding to the two-sub Mach-Zehnder waveguide.

In a second aspect of the present invention, in the laser device of the first aspect, a part of the frequency modulated laser light outputted by the laser device is used as a reference light, and at least a part of the remaining part is branched as a measurement light. a branching section; a beat signal generating section that generates a beat signal by mixing the reflected light reflected by irradiating the measurement light onto the measurement target and the reference light; and frequency-analyzing the beat signal. A measuring device is provided, including a detection unit that detects a difference in propagation distance between the reference light and the measurement light.

In a second aspect of the present invention, in the laser device of the first aspect, a part of the frequency modulated laser light outputted by the laser device is used as a reference light, and at least a part of the remaining part is branched as a measurement light. a branching section; a beat signal generating section that generates a beat signal by mixing the reflected light reflected by irradiating the measurement light onto the measurement target and the reference light; and frequency-analyzing the beat signal. A measuring device is provided, including a detection unit that detects a difference in propagation distance between the reference light and the measurement light.

In a third aspect of the present invention, in the laser device of the first aspect, a part of the frequency modulated laser light outputted by the laser device is used as a reference light, and at least a part of the remaining part is branched as a measurement light. a branching section; a beat signal generating section that generates a beat signal by mixing the reflected light reflected by irradiating the measurement light onto the measurement target and the reference light; and frequency-analyzing the beat signal. A measuring device is provided, including a detection unit that detects a difference in propagation distance between the reference light and the measurement light.

The control unit sets the frequency shift amount of the optical SSB modulator to either a first frequency or a second frequency larger than the first frequency, and then causes the detection unit to frequency-analyze the beat signal. and the detection unit detects an order determination beat signal generated when the frequency shift amount of the optical SSB modulator is set to the first frequency and the frequency shift amount of the optical SSB modulator to the second frequency. The difference in propagation distance between the reference light and the measurement light may be detected based on the results of frequency analysis of the distance measurement beat signals generated when the reference light and the measurement light are used.

The control section switches the frequency shift direction of the optical SSB modulator to either the positive side or the negative side, and then causes the detection section to frequency-analyze the beat signal. Frequency analysis is performed on a positive beat signal generated when the frequency shift direction of the modulator is set to the positive side, and a negative side beat signal generated when the frequency shift direction of the optical SSB modulator is set to the negative side. Based on the result, a difference in propagation distance between the reference light and the measurement light may be detected.

The beat signal generator may perform orthogonal detection of the reflected light and the reference light.

In a fourth aspect of the present invention, the step of setting the frequency shift direction of the optical SSB modulator to either the positive side or the negative side, and comprising an amplification medium and the optical SSB modulator in a laser resonator. outputting a plurality of modes of frequency modulated laser light from a laser device; a step of making a part of the frequency modulated laser light output by the laser device a reference light and branching at least part of the remaining part as a measurement light; generating a positive beat signal by irradiating the measurement light onto the measurement target and mixing the reflected light with the reference light; and changing the frequency shift direction of the optical SSB modulator to the opposite direction. and detecting a difference in propagation distance between the reference light and the measurement light based on the results of frequency analysis of the positive beat signal and the negative beat signal. Provided is a measurement method comprising:

In a fifth aspect of the present invention, the step of setting the frequency shift amount of an optical SSB modulator to a first frequency, and the step of setting a frequency shift amount of an optical SSB modulator to a first frequency, outputting a frequency modulated laser beam, a step of using a part of the frequency modulated laser beam outputted by the laser device as a reference beam, and branching at least a part of the remaining part as a measurement beam; generating an order determination beat signal by mixing reflected light irradiated onto an object and reflected by the reference light; and detecting a difference in propagation distance between the reference light and the measurement light based on the result of frequency analysis of the order determination beat signal and the distance measurement beat signal. Provided is a measuring method comprising steps.

In the sixth aspect of the present invention, the step of setting the frequency shift amount of the optical SSB modulator to the first frequency and setting the frequency shift direction to either the positive side or the negative side; outputting a plurality of modes of frequency modulated laser light from a laser device having an amplification medium and the optical SSB modulator; a step of branching a part of the measurement light as a measurement light; a step of generating an order determination beat signal by mixing the reference light with a reflected light that is reflected by irradiating the measurement light onto the measurement object; and a step of generating an order determination beat signal by switching the frequency shift amount of the modulator to a second frequency larger than the first frequency to generate a positive beat signal; and keeping the frequency shift amount of the optical SSB modulator at the second frequency; switching the frequency shift direction to the opposite direction to generate a negative beat signal; and adjusting the reference light and the measurement light based on the order determination beat signal, the positive beat signal, and the negative beat signal. and detecting a difference in propagation distance between the two.

According to the present invention, it is possible to easily change the shift amount and/or the shift direction for shifting the frequency of the frequency-shifted feedback laser.

A configuration example of a laser device 110 according to the present embodiment is shown. An example of the configuration of the AOFS 20 is shown. A configuration example of an optical SSB modulator 30 and a control unit 50 according to the present embodiment is shown. An example of laser light output by the laser device 110 according to the present embodiment is shown. A configuration example of a measuring device 100 according to this embodiment is shown together with a measurement target 10. An example of the relationship between the frequency of the beat signal detected by the measuring device 100 according to the present embodiment and the distance d between the optical head section 140 and the measurement target object 10 is shown. A configuration example of a beat signal generation section 150 and a detection section 160 according to the present embodiment is shown. An example of the outline of orthogonal detection of the beat signal generating section 150 and the detecting section 160 according to the present embodiment is shown. A first example of the operation flow of the measuring device 100 according to the present embodiment is shown. The concept of the optical frequencies of the reference light and measurement light of the measuring device 100 according to this embodiment, and the frequency of the observed beat signal is shown. A second example of the operation flow of the measuring device 100 according to the present embodiment is shown. The concept of the relationship between the frequency shift amount ν _s and the absolute length measurement range of the laser device 110 according to the present embodiment is illustrated. A third example of the operation flow of the measuring device 100 according to the present embodiment is shown. A modified example of the control unit 50 according to the present embodiment is shown together with the optical SSB modulator 30.

[Example of configuration of laser device 110]
FIG. 1 shows a configuration example of a laser device 110 according to this embodiment. A laser device 110 shown in FIG. 1 is an example of a frequency-shifted feedback laser that outputs frequency-modulated laser light in a plurality of modes. The laser device 110 has a laser resonator and oscillates a laser beam within the laser resonator. The laser resonator of the laser device 110 includes a frequency shifter 112 , an amplification medium 114 , a WDM coupler 116 , a pump light source 117 , and an output coupler 118 .

The frequency shifter 112 shifts the frequency of input light by a substantially constant frequency. Here, the amount of frequency shift by the frequency shifter 112 is assumed to be +ν _s or −ν _s . For example, the frequency shifter 112 shifts the frequency of light circulating around the resonator so that the frequency increases by ν _s for each revolution.

The amplification medium 114 is supplied with pump light and amplifies the input light. The amplification medium 114 is, for example, an optical fiber doped with impurities. The impurity is, for example, a rare earth element such as erbium, neodymium, ytterbium, terbium, or thulium. Further, pump light is supplied to the amplification medium 114 from a pump light source 117 via a WDM coupler 116 . The output coupler 118 outputs a portion of the laser oscillated light within the resonator to the outside.

That is, the laser device 110 shown in FIG. 1 constitutes a fiber ring laser having a frequency shifter 112 within a resonator. It is desirable that the laser device 110 further include an isolator within the resonator. Further, the laser device 110 may include an optical bandpass filter in the resonator that passes light in a predetermined wavelength band.

As the frequency shifter 112 used in such a laser device 110, an AOFS (Acousto-Optic Frequency Shifter) having an acousto-optic element is conventionally known. AOFS will be explained next.

[Configuration example of AOFS20]
FIG. 2 shows a configuration example of the AOFS 20. The AOFS 20 includes an optical crystal 21, an ultrasonic transducer 22, an ultrasonic absorber 23, and an oscillator 24. The optical crystal 21 is a crystal that transmits light, and is made of, for example, TeO ₂ , PbMoO ₄ , heavy flint glass, or the like.

The ultrasonic transducer 22 is provided on the end face of the optical crystal 21 and generates ultrasonic waves according to the electrical signal of frequency ω ₁ supplied from the oscillator 24 . The ultrasonic transducer 22 propagates generated ultrasonic waves, for example, from one end surface of the optical crystal 21 to an end surface opposite to the other end surface. In FIG. 2, an example of the propagation direction of ultrasonic waves is indicated by an arrow. Note that an ultrasonic absorber 23 is provided on the end surface of the optical crystal 21 that is opposite to the end surface on which the ultrasonic transducer 22 is provided.

Refractive index modulation occurs inside the optical crystal 21 due to the propagation of ultrasonic waves. When light is made incident on such an optical crystal 21, the incident light is diffracted by Raman-Nath diffraction. The frequency of the diffracted light is shifted from the frequency ω ₀ of the incident light by the frequency ω ₁ of the ultrasound due to the Doppler effect due to the propagating ultrasound. In this way, the frequency of the diffracted light shifts based on the Doppler effect, so the sign of the shifted frequency is determined depending on the propagation direction of the ultrasonic wave and the direction of incidence of the incident light. FIG. 2 shows an example in which the direction of the frequency shift is positive and the frequency of the diffracted light is ω ₀ +ω ₁ .

For example, in order to generate strong diffracted light, the refractive index grating spacing d by the ultrasound, the wavelength λ of the light wave, the incident angle θi of the light wave, the diffraction angle θo, and the order of diffraction h are d(sinθi−sinθo)=hλ, h=0, ±1, ±2, . ．． ．． It is necessary to satisfy the following relationship. If h is positive, the frequency shift will be positive, if h is negative, the frequency shift will be negative, and if h is 0, no frequency shift will occur. Generally, λ and θi are fixed, and θo is adjusted so as to extract diffracted light with h either +1 or -1.

Therefore, in order to switch the positive or negative of the shifted frequency in the AOFS 20, for example, the propagation direction of the ultrasound must be switched to the opposite direction, or the incident angle of the light must be changed. Since the propagation direction of ultrasonic waves and the incident angle of light cannot be easily changed by electric signals or the like, it is difficult to switch the frequency shift direction in the conventional laser device 110 using such an AOFS 20. Ta.

Further, since the ultrasonic transducer 22 has a resonant frequency due to its material, it is not possible to generate ultrasonic waves with a frequency that is significantly different from the resonant frequency. Further, the diffraction angle of the light diffracted by the optical crystal 21 is an angle corresponding to the frequency of the ultrasonic wave. If the diffraction angle changes depending on the amount of frequency shift, a coupling loss with the output side optical system will occur. Therefore, in the AOFS 20, it has been difficult to freely change the shift amount of the frequency to be shifted. Therefore, by using an optical SSB modulator as the frequency shifter 112, the laser device 110 according to the present embodiment can easily switch the frequency shift amount and/or shift direction. Such an optical SSB modulator will be explained next.

[Configuration example of optical SSB modulator 30]
FIG. 3 shows a configuration example of the optical SSB modulator 30 and the control section 50 according to this embodiment. The optical SSB modulator 30 includes a substrate 31 , a main Mach-Zehnder waveguide 32 , a first sub-Mach-Zehnder waveguide 33 , a second sub-Mach-Zehnder waveguide 34 , a main DC electrode 35 , a first sub-DC electrode 36 , and a second sub-DC electrode 37 , a first RF electrode 38, and a second RF electrode 39.

The substrate 31 is a substrate at least partially made of electro-optic crystal, and includes, for example, LiNbO ₃ crystal. A waveguide and a substrate are formed on the surface of such a substrate 31. The main Mach-Zehnder waveguide 32 branches the light input to the optical SSB modulator 30 into two, combines the branched lights again, and then outputs them. The main Mach-Zehnder waveguide 32 has a first arm waveguide 41 and a second arm waveguide 42 that respectively pass the two branched lights.

The first arm waveguide 41 is provided with a first sub-Mach-Zehnder waveguide 33 . The first sub-Mach-Zehnder waveguide 33 branches the light that the first arm waveguide 41 passes into two, combines the branched lights again, and then outputs the light to the first arm waveguide 41 . The first sub-Mach-Zehnder waveguide 33 includes a first sub-arm waveguide 43 and a second sub-arm waveguide 44 through which input light passes.

The second arm waveguide 42 is provided with a second sub-Mach-Zehnder waveguide 34 . The second sub-Mach-Zehnder waveguide 34 branches the light that the second arm waveguide 42 passes into two, combines the branched lights again, and then outputs the light to the second arm waveguide 42 . The second sub-Mach-Zehnder waveguide 34 has a third sub-arm waveguide 45 and a fourth sub-arm waveguide 46 through which input light passes.

For example, the main DC electrode 35 is provided at a position approximately the same distance from each of the first arm waveguide 41 and the second arm waveguide 42 of the main Mach-Zehnder waveguide 32. The main DC electrode 35 is supplied with a DC voltage from the control unit 50 .

For example, the first sub-DC electrode 36 and the first RF electrode 38 are provided at positions approximately the same distance away from the first sub-arm waveguide 43 and the second sub-arm waveguide 44 of the first sub-Mach-Zehnder waveguide 33, respectively. It is being The first sub-DC electrode 36 and the first RF electrode 38 may be separate electrodes or may be one common electrode.

Similarly, the second sub-DC electrode 37 and the second RF electrode 39 are, for example, approximately the same distance away from the third sub-arm waveguide 45 and the fourth sub-arm waveguide 46 of the second sub-Mach-Zehnder waveguide 34. located at the location. The second sub-DC electrode 37 and the second RF electrode 39 may be separate electrodes or may be one common electrode.

A DC voltage is supplied from the control unit 50 to the first sub-DC electrode 36 and the second sub-DC electrode 37, respectively. An RF signal is supplied from the control unit 50 to the first RF electrode 38 and the second RF electrode 39, respectively. The RF signal is, for example, a high frequency signal from several GHz to several tens of GHz.

In this way, by applying a voltage to the electrodes provided near the waveguide through which input light passes, an electro-optic effect (Pockels effect) that changes the refractive index of the waveguide occurs. Modulation, offset, etc. corresponding to the applied voltage will occur in the amplitude intensity level and phase of the light passing through the waveguide where the electro-optic effect is occurring. Since the change in refractive index due to such an electro-optic effect corresponds to the direction of application of the electric field, the direction of change in phase can be switched, for example, simply by changing the sign of the voltage applied to the electrodes.

The control unit 50 supplies a DC voltage and an RF signal to the plurality of electrodes of the optical SSB modulator 30 to adjust the phase of light passing through the waveguide. The control section 50 includes a DC voltage generation section 52 and an RF signal generation section 54. The DC voltage generator 52 generates a DC voltage and supplies it to the main DC electrode 35 , the first sub DC electrode 36 , and the second sub DC electrode 37 . The RF signal generation section 54 generates an RF signal and supplies it to the first RF electrode 38 and the second RF electrode 39.

The control unit 50 controls the DC voltage generation unit 52 and the RF signal generation unit 54 to supply a DC voltage and an RF signal to the optical SSB modulator 30, and adjusts the frequency shift direction and frequency shift amount. For example, the control unit 50 shifts the frequency of light input to the optical SSB modulator 30 by the frequency of the RF signal. The control unit 50 may be able to further set the frequency shift amount of the optical SSB modulator 30 by changing the frequency of the RF signal.

Further, the control unit 50 controls the main DC electrode 35 and the first sub DC electrode provided on the substrate 31 corresponding to the main Mach Zehnder waveguide 32, the first sub Mach Zehnder waveguide 33, and the second sub Mach Zehnder waveguide 34. 36 and the second sub-DC electrode 37, a DC voltage of a predetermined value is supplied to switch the frequency shift direction. Such frequency shifting and switching of the shift direction of the optical SSB modulator 30 are described in known Patent Documents 2 and 3, and detailed explanations thereof will be omitted here.

As described above, the laser device 110 according to this embodiment uses the optical SSB modulator 30 shown in FIG. 3 as the frequency shifter 112. The control unit 50 can set the frequency shift amount of the optical SSB modulator 30 by setting the frequency of the RF signal supplied to the optical SSB modulator 30. Further, the control unit 50 can switch the frequency shift direction of the optical SSB modulator 30 to either the positive side or the negative side by switching the voltage supplied to the optical SSB modulator 30. Thereby, the shift amount and positive/negative direction for shifting the frequency of the laser device 110 can be easily set. Next, the frequency characteristics of the laser light outputted by such a laser device 110 will be explained.

FIG. 4 shows an example of laser light output by the laser device 110 according to this embodiment. FIG. 4 shows the optical spectrum of the laser beam output by the laser device 110 at time t ₀ on the left side. In the optical spectrum, the horizontal axis represents the light intensity, and the vertical axis represents the frequency of the light. Further, a plurality of longitudinal modes of the optical spectrum are indicated by a number q. The frequencies of the plurality of longitudinal modes are arranged at substantially constant frequency intervals. Here, if the time for light to go around the resonator once is τ _RT (=1/ν _C ), then the multiple longitudinal modes are arranged at intervals of 1/τ _RT (=ν _C ) as shown in the following equation. Become. Note that ν ₀ is the initial frequency of the optical spectrum at time t ₀ .

FIG. 4 shows, on the right side, changes in frequency over time of a plurality of longitudinal modes output by the laser device 110. On the right side of FIG. 4, the horizontal axis represents time and the vertical axis represents frequency. That is, in FIG. 4, the right side shows temporal changes in the frequency of the laser beam output by the laser device 110, and the left side shows the instantaneous frequency of the laser beam at time _t0 .

For example, in the laser device 110, the shift amount of the frequency shifter 112 is set to +ν _s , and each time the light inside the resonator goes around the resonator once, the frequency shifter 112 increases the frequency of the circulating light by +ν _s . let That is, each time τ _RT elapses, the frequency of each mode increases by ν _s , so the time change in frequency dν/dt becomes approximately equal to ν _s /τ _RT . Therefore, the plurality of longitudinal modes expressed by equation (1) change as shown in the following equation as time t passes.

Note that Equation (2) is an equation when the frequency shift direction is set to the plus direction (positive direction). On the right side of FIG. 4, an example of a temporal change in the frequency of the laser beam when the frequency shift direction is set in the positive direction is shown in the period between time t ₀ and t ₁ . Alternatively, if the frequency shift direction is set to the minus direction (negative direction), the sign of ν _s ·t/τ _RT , which is the second term in equation (2), becomes negative. On the right side of FIG. 4, an example of a temporal change in the frequency of the laser light when the frequency shift direction is set in the negative direction is shown in the period between time _t2 and _t3 .

As described above, the laser device 110 using the optical SSB modulator 30 as the frequency shifter 112 can switch the frequency shift direction to either the positive direction or the negative direction. Further, the laser device 110 can easily change the frequency shift amount. The optical SSB modulator 30 can change the frequency shift amount and shift direction without changing the output angle. That is, the laser device 110 can easily change the frequency shift amount and shift direction without substantially changing the optical output. When such a laser device 110 is used in an optical distance meter or the like, it is possible to easily improve the accuracy of distance measurement and improve the function. Therefore, an optical rangefinder including the laser device 110 according to this embodiment will be described next.

[Configuration example of measuring device 100]
FIG. 5 shows a configuration example of the measuring device 100 according to the present embodiment together with the measurement target 10. As shown in FIG. The measuring device 100 optically measures the distance between the measuring device 100 and the object to be measured 10 . Furthermore, the measuring device 100 may measure the three-dimensional shape of the measurement object 10 by scanning the position of the laser beam irradiated onto the measurement object 10. The measurement device 100 includes a control section 50, a laser device 110, a branching section 120, an optical circulator 130, an optical head section 140, a beat signal generation section 150, a detection section 160, and a display section 170.

The control unit 50 supplies a DC voltage and an RF signal for controlling the laser device 110. Laser device 110 is a frequency-shifted feedback laser with optical SSB modulator 30 . Since the control unit 50 and the laser device 110 have already been explained, their explanation will be omitted here.

The branching unit 120 branches a part of the frequency modulated laser beam outputted by the laser device 110 into a reference light, and branches at least a part of the remaining part into a measurement light. The branching section 120 is, for example, an optical fiber type optical coupler. In the example of FIG. 5, the branching section 120 supplies the measurement light to the optical circulator 130 and the reference light to the beat signal generation section 150.

Optical circulator 130 has multiple input/output ports. For example, the optical circulator 130 causes light input to one port to be output from the next port, and light input from the next port to be output from the next port. FIG. 5 shows an example in which the optical circulator 130 has three input/output ports. In this case, the optical circulator 130 outputs the measurement light supplied from the branching section 120 to the optical head section 140. Further, the optical circulator 130 outputs the light input from the optical head section 140 to the beat signal generating section 150.

The optical head unit 140 irradiates the measurement target object 10 with light input from the optical circulator 130 . The optical head section 140 includes, for example, a collimator lens. In this case, the optical head unit 140 adjusts the light input from the optical circulator 130 via the optical fiber into a beam using a collimator lens, and then outputs the beam.

Further, the optical head section 140 receives reflected light of the measurement light irradiated onto the measurement target object 10. The optical head section 140 condenses the received reflected light onto an optical fiber using a collimator lens, and supplies it to the optical circulator 130 . In this case, the optical head section 140 has one common collimator lens, and the collimator lens may irradiate measurement light onto the measurement object 10 and receive reflected light from the measurement object 10. . Note that the distance between the optical head section 140 and the measurement target object 10 is assumed to be d.

Alternatively, the optical head section 140 may include a condenser lens. In this case, the optical head section 140 focuses the light input from the optical circulator 130 via the optical fiber onto the surface of the measurement target object 10. The optical head section 140 receives at least a portion of the reflected light reflected from the surface of the measurement target object 10. The optical head section 140 condenses the received reflected light onto an optical fiber using a condensing lens and supplies it to the optical circulator 130 . In this case as well, the optical head section 140 has one common condensing lens, which irradiates the measuring object 10 with measurement light and also collects the reflected light from the measuring object 10. May receive light.

The beat signal generating section 150 irradiates the measurement object 10 with measurement light and receives the reflected light from the optical circulator 130 . Furthermore, the beat signal generating section 150 receives the reference light from the branching section 120. The beat signal generator 150 mixes the reflected light and the reference light to generate a beat signal. The beat signal generating section 150 includes, for example, a photoelectric conversion element, converts a beat signal into an electrical signal, and outputs the electrical signal.

Here, since the reflected light travels back and forth over the distance from the optical head section 140 to the measurement target object 10, a difference in propagation distance will occur compared to the reference light at least according to the distance 2d. Since the oscillation frequency of the light output from the laser device 110 changes linearly over time, a frequency difference occurs between the oscillation frequencies of the reference light and the reflected light in accordance with the propagation delay corresponding to the difference in propagation distance. The beat signal generating section 150 generates a beat signal corresponding to such a frequency difference.

The detection unit 160 analyzes the frequency of the beat signal generated by the beat signal generation unit 150 and detects a difference in propagation distance between the reference light and the measurement light. Frequency analysis by the detection unit 160 will be described later.

The display section 170 displays the analysis results of the detection section 160. The display unit 170 may include a display or the like and display the detection results. Further, the display unit 170 may store the analysis results in a storage unit or the like.

The above measurement device 100 can measure the distance between the measurement device 100 and the measurement object 10 by analyzing the frequency difference between the reflected light of the measurement light irradiated on the measurement object 10 and the reference light. shall be. That is, the measuring device 100 can constitute a non-contact and non-destructive optical distance meter.

[Details of distance measurement processing]
The measuring device 100 according to the present embodiment measures the distance d between the optical head section 140 and the measurement target 10 using the laser device 110 that outputs a frequency component as shown in equation (2). Here, the optical path difference between the reference light and the reflected light is only a distance 2d traveling back and forth over the distance d, and the propagation delay corresponding to the distance 2d is assumed to be Δt. That is, when the measurement light is reflected from the measurement target 10 and returned at time t, the returned reflected light approximately matches the frequency of the past by time Δt than time t, so it can be expressed by the following equation. Can be done.

On the other hand, the reference light at time t can be expressed by the following equation, similar to equation (2). Here, the reference light is ν _q' (t).

Since the beat signal generator 150 superimposes such reflected light and reference light, it generates a plurality of beat signals between the plurality of longitudinal modes expressed by Equation (3) and the plurality of longitudinal modes expressed by Equation (4). will occur. Letting the frequency of such a beat signal be ν _B (m, d), ν _B (m, d) can be expressed by the following equation from equations (3) and (4). Note that m is the interval between longitudinal mode numbers (=q-q'), and Δt=2d/c.

From the equation (5), the distance d is expressed as the following equation. Here, 1/τ _RT =ν _c .

From equation (6), it can be seen that the distance d can be calculated from the frequency observation result of the beat signal by determining the interval m of the longitudinal mode numbers. Note that the interval m is called the order m of the beat signal, and can be determined by detecting a change in the beat signal when the frequency shift amount ν _s of the laser device 110 is changed. Since such a method for determining the order m is known as described in Patent Document 1 and the like, detailed explanation will be omitted here.

Since the observed beat signal always has a positive frequency, the beat signal that occurs on the negative frequency side is calculated to be folded back to the positive side and is observed as an image signal. Generation of such an image signal will be explained next.

FIG. 6 shows an example of the relationship between the frequency of the beat signal detected by the measuring device 100 according to this embodiment and the distance d between the optical head section 140 and the measurement target object 10. The horizontal axis in FIG. 6 indicates the distance d, and the vertical axis indicates the frequency ν _B (m, d) of the beat signal. The plurality of straight lines shown by the solid lines in FIG. 6 are graphs showing the relationship between the frequency ν _B (m, d) of the beat signal and the distance d for each of the plurality of orders m, as shown in equation (5). be.

As shown in FIG. 6, a plurality of beat signals are generated depending on the value of m. However, since the plurality of longitudinal modes contained in each of the reflected light and the reference light are arranged at a substantially constant frequency interval ν _c , the plurality of beat signals with the same value of m are superimposed at substantially the same frequency on the frequency axis. That will happen. For example, when observing a frequency band between frequencies 0 and _νc , a plurality of beat signals are superimposed on substantially the same frequency and are observed as a single line spectrum.

In addition to this, the absolute value of the frequency ν _B (m, d) of the beat signal in the negative region smaller than 0 is further observed as an image signal. That is, the graph in the area where the vertical axis of FIG. 6 is smaller than 0 is folded back with frequency 0 as the boundary. FIG. 6 shows folded image signals with a plurality of dotted lines. Since the folded image signals are simply reversed in polarity, they are superimposed on the observed frequency axis at the same frequency as the absolute value of the frequency before folding. For example, when observing a frequency band between frequencies 0 and ν _c , such beat signals and image signals are located at different frequencies unless their respective frequencies are ν _c /2.

In this way, in the observation band between frequencies 0 and ν _C , the beat signal ν _B (m, d) and the image signal _{ν B} (m, d) have different values of m. ', d) two line spectra are generated. Here, as an example, m'=m+1. In this case, such an image signal can be canceled by the beat signal generating section 150 using quadrature detection. Therefore, the beat signal generating section 150 and the detecting section 160 using orthogonal detection will be explained next.

[Configuration example of beat signal generation section 150 and detection section 160]
FIG. 7 shows a configuration example of the beat signal generation section 150 and the detection section 160 according to this embodiment. The beat signal generator 150 orthogonally detects the reflected light and the reference light. The beat signal generation section 150 includes an optical 90-degree hybrid 152 and two photoelectric conversion sections 154.

The optical 90-degree hybrid 152 branches each input reflected light and reference light into two. The optical 90-degree hybrid 152 generates a first beat signal by combining one of the branched reflected lights and one of the branched reference lights using an optical coupler or the like. Further, the optical 90-degree hybrid 152 generates a second beat signal by combining the other branched reflected light and the other branched reference light using an optical coupler or the like. Here, the optical 90-degree hybrid 152 generates a beat signal after creating a 90-degree phase difference between the two branched reference lights. For example, the optical 90-degree hybrid 152 combines one of the two branched reference lights with the reflected light after passing through a π/2 wavelength plate.

The photoelectric conversion unit 154 receives the combined reflected light and reference light and converts them into electrical signals. The photoelectric conversion unit 154 may be a photodiode or the like. The photoelectric conversion unit 154 is, for example, a balanced photodiode. In FIG. 7, it is assumed that one of the two photoelectric conversion units 154 generates a first beat signal, and the other photoelectric conversion unit 154 generates a second beat signal. As described above, the beat signal generating section 150 multiplexes the two reference lights and the reflected light whose phases are different by 90 degrees, performs orthogonal detection, and outputs two beat signals to the detecting section 160.

The detection unit 160 performs frequency analysis on the two beat signals. Here, an example will be described in which the detection unit 160 performs frequency analysis using the first beat signal as the I signal and the second beat signal as the Q signal. The detection section 160 includes a first filter section 162, a second filter section 164, a first AD conversion section 202, a second AD conversion section 204, a clock signal supply section 210, and a frequency analysis section 220.

The first filter section 162 and the second filter section 164 reduce signal components in a frequency band different from the frequency band that the user or the like desires to perform frequency analysis. Here, it is assumed that the frequency band that the user or the like desires to perform frequency analysis is from 0 to _νc . The first filter section 162 and the second filter section 164 are, for example, low-pass filters that pass signal components having a frequency ν _c or lower. In this case, the first filter section 162 supplies the first AD conversion section 202 with a first beat signal in which signal components of frequencies higher than the frequency ν _c are reduced. Furthermore, the second filter section 164 supplies the second AD conversion section 204 with a second beat signal in which signal components of frequencies higher than the frequency ν _c are reduced.

The first AD converter 202 and the second AD converter 204 convert input analog signals into digital signals. For example, the first AD converter 202 converts the first beat signal into a digital signal, and the second AD converter 204 converts the second beat signal into a digital signal. The clock signal supply section 210 supplies a clock signal to the first AD conversion section 202 and the second AD conversion section 204. Thereby, the first AD converter 202 and the second AD converter 204 convert the analog signal into a digital signal at a sampling rate that is substantially the same as the frequency of the received clock signal.

Here, if the observation band is from 0 to ν _c , the frequency of the beat signal is at most the resonant frequency ν _c of the laser resonator. Therefore, the clock signal supply section 210 supplies the first AD conversion section 202 and the second AD conversion section 204 with a clock signal having a frequency that is twice or more the resonance frequency ν _c of the laser resonator, thereby observing the beat signal. Can be done. Here, a frequency that is twice or more the resonance frequency ν _c is defined as a sampling frequency. In this manner, the detection unit 160 performs frequency analysis on the first beat signal and second beat signal generated by sampling the beat signal at the sampling frequency, using the first beat signal and the second beat signal as first sampling data.

The frequency analysis section 220 converts the first beat signal and the second beat signal into frequency data. For example, the frequency analysis unit 220 performs digital Fourier transform (DFT) on each of the first beat signal and the second beat signal. The frequency analysis unit 220 adds the first beat signal converted into frequency data as a real part and the second beat signal converted into frequency data as an imaginary part, thereby canceling out the image signal. The above operation of the detection section 160 is controlled by the control section 50, for example.

At least a portion of the control section 50 and the detection section 160 are configured of, for example, an integrated circuit or the like. In this case, a portion of the detection unit 160 after the beat signal is converted into a digital signal is configured with an integrated circuit or the like. For example, the control unit 50 and the detection unit 160 include an FPGA (Field Programmable Gate Array), a DSP (Digital Signal Processor), and/or a CPU (Central Processing Unit).

When at least a portion of the control section 50 and the detection section 160 are configured by a computer or the like, the control section 50 and the detection section 160 include a storage section. The storage unit includes, for example, a ROM (Read Only Memory) that stores the BIOS (Basic Input Output System) of a computer etc. that implements the control unit 50 and the frequency analysis unit 220, and a RAM (Random Access Memory) that serves as a work area. including. Further, the storage unit may store various information including an OS (Operating System), an application program, and/or a database referenced when the application program is executed. That is, the storage unit may include a mass storage device such as an HDD (Hard Disk Drive) and/or an SSD (Solid State Drive).

A computer or the like includes a processor such as a CPU, and functions as the control section 50 and the frequency analysis section 220 by executing a program stored in a storage section. The computer or the like may include a GPU (Graphics Processing Unit) or the like.

[Outline of orthogonal detection]
FIG. 8 schematically shows an example of quadrature detection by the beat signal generation section 150 and the detection section 160 according to the present embodiment. In FIG. 8, the horizontal axis shows the frequency of the beat signal, and the vertical axis shows the signal strength. FIG. 8 shows the frequency spectrum of either the I signal or the Q signal. The frequency spectra of both the I signal and the Q signal have substantially the same spectral shape, as shown in the upper part of FIG. As for the I signal and the Q signal, for example, a beat signal ν _B (m, d) and an image signal ν _B (m+1, d) are observed in a frequency band between frequency 0 and ν _c . In this case, the I signal and the Q signal include the beat signal -ν _B (m, _d ) and the original beat signal of the image signal -ν _B (m+1 , d) exists.

Here, since the I signal and the Q signal are signal components orthogonally detected by the beat signal generating section 150, they contain different phase information even if their spectrum shapes are the same. For example, in the frequency band between frequencies 0 and ν _c on the positive side, the image signals ν _B (m+1, d) of the I signal and the Q signal have mutually inverted phases. Similarly, in the frequency band from frequency 0 to -v _c on the negative side, the beat signals -v _B (m, d) of the I signal and the Q signal are inverted in phase.

Therefore, as shown in the lower part of FIG. 8, when the frequency analysis unit 220 calculates I+jQ using the I signal and the Q signal, the frequency ν _B (m, d) in the frequency band from frequency 0 to ν _c The beat signals of ν B (m+1, d) are reinforced, and the image signal of frequency ν _B (m+1, d) is canceled out. Similarly, in the frequency band between frequency 0 and -ν _c , two beat signals with frequency -ν _B (m+1, d) strengthen each other, and two beat signals with frequency -ν _B (m, d) cancel each other out. be done.

According to the frequency analysis result of the frequency analysis unit 220, one beat signal is observed at frequency ν _B (m, d) in the frequency band between frequencies 0 and ν _c . Since the measuring device 100 can cancel the image signal in this way, it can measure the distance d between the optical head section 140 and the measurement target object 10.

As described above, according to the measuring device 100 according to the present embodiment, the distance d between the measuring device 100 and the object to be measured 10 can be measured in a non-destructive and non-contact manner. The measuring device 100 that measures the distance d to the measurement target 10 in a non-contact manner is often arranged separately and independently from the measurement target 10. Therefore, when at least one of the measuring device 100 and the measuring object 10 is vibrating or moving, a relative velocity will occur between the measuring device 100 and the measuring object 10. Such relative speed may affect the measurement results of the measuring device 100 as errors.

For example, if the relative velocity component of the measurement light of the measuring device 100 in the direction parallel to the optical axis direction has a finite value, the reflected light from the measurement target 10 will have a certain value depending on the relative velocity component due to the Doppler effect. A frequency shift occurs. The frequency shift of the reflected light changes the frequency of the beat signal generated by the beat signal generating section 150, which causes an error in the measurement results of the measuring device 100.

Therefore, the measuring device 100 according to the present embodiment changes the frequency shift direction of the laser device 110 and performs frequency analysis on the beat signals before and after changing the shift direction, thereby reducing errors based on such Doppler effect. . The measurement operation of such a measurement device 100 will be explained next.

[First example of operation flow of measuring device 100]
FIG. 9 shows a first example of the operation flow of the measuring device 100 according to this embodiment. The measuring device 100 measures the distance d between the optical head section 140 and the measurement target object 10 by executing the operations from S1010 to S1060 in FIG.

First, in S1010, the control unit 50 sets the frequency shift direction of the optical SSB modulator 30 to either the positive side or the negative side. For example, the control unit 50 sets the frequency shift direction of the optical SSB modulator 30 to the positive side. Further, the control unit 50 may further set the shift amount ν _s of the frequency shift of the optical SSB modulator 30.

Next, in S1020, the control unit 50 controls the laser device 110 having the amplification medium 114 and the optical SSB modulator 30 in the laser resonator to output frequency modulated laser light of a plurality of modes. Then, the branching unit 120 branches part of the frequency modulated laser light outputted by the laser device 110 into a reference light, and branches at least part of the remaining part into a measurement light. The optical head section 140 irradiates the measurement object 10 with measurement light. The optical head section 140 then receives the reflected light from the measurement target object 10. The beat signal generator 150 mixes the reflected light and the reference light to generate a positive beat signal.

Next, in S1030, the control unit 50 controls the detection unit 160 to cause the detection unit 160 to analyze the frequency of the positive beat signal. For example, the detection unit 160 samples the positive beat signal at a sampling frequency that is twice or more the resonance frequency of the laser resonator to generate sampling data. The frequency analysis unit 220 calculates I+jQ by frequency converting the I signal and Q signal of the sampling data. Then, the frequency analysis unit 220 calculates the positive frequency _F1 at which the positive beat signal is generated.

Next, in S1040, the control unit 50 switches the frequency shift direction of the optical SSB modulator 30 to the opposite direction. For example, the control unit 50 switches the frequency shift direction of the optical SSB modulator 30 from the positive side to the negative side. The branching unit 120 branches a portion of the frequency modulated laser beam whose frequency shift direction has been switched as reference light, and branches at least a portion of the remaining portion as measurement light. The optical head section 140 irradiates the measurement object 10 with measurement light. The optical head section 140 receives reflected light reflected from the measurement target object 10. Then, the beat signal generating section 150 mixes the reflected light and the reference light to generate a negative beat signal.

Next, in S1050, the control unit 50 switches the frequency shift direction of the optical SSB modulator 30, and then controls the detection unit 160 to frequency-analyze the negative beat signal. For example, similarly to the positive beat signal, the detection unit 160 calculates the negative frequency _F2 at which the negative beat signal is generated.

Next, in S1060, the frequency analysis unit 220 detects the difference in propagation distance between the reference light and the measurement light, that is, the distance d, based on the results of frequency analysis of the positive beat signal and the negative beat signal. Here, if a Doppler effect occurs depending on the relative speed between the measurement device 100 and the measurement target 10, the frequency analysis results of the positive beat signal and the negative beat signal will include errors. .

For example, if the frequency error due to the Doppler effect is ΔF, the positive frequency F ₁ of the positive beat signal can be expressed as F ₀ −ΔF. Here, F ₀ is the frequency of the positive beat signal observed when the Doppler effect does not occur. Furthermore, since the negative side frequency F ₂ of the negative side beat signal has the frequency shift direction of the optical SSB modulator 30 switched to the opposite side, the sign of the error due to the Doppler effect is reversed and can be expressed as F ₀ +ΔF. Can be done. The concept of such positive side frequency F ₁ and negative side frequency F ₂ will be explained using FIG. 10.

FIG. 10 shows the concept of the optical frequencies of the reference light and measurement light of the measuring device 100 according to this embodiment, and the frequency of the observed beat signal. In the upper diagram of FIG. 10, the horizontal axis indicates time, and the vertical axis indicates the optical frequencies of the reference light and measurement light. Further, in the lower diagram of FIG. 10, the horizontal axis represents time, and the vertical axis represents the frequency of the beat signal. FIG. 10 shows an example in which the control unit 50 repeatedly switches the frequency shift direction of the optical SSB modulator 30 at a constant period T.

For example, the reference light that has reached the beat signal generator 150 during the period from time t _n to t _n+1 is the same as the laser light output from the laser device 110 during the period when the frequency shift direction of the optical SSB modulator 30 is set to the positive side. Part of it. Furthermore, the measurement light branched during the period when the frequency shift direction of the optical SSB modulator 30 is set to the positive side reaches the beat signal generation unit 150 during the period from time t _m to t _m+1 delayed by a certain time. .

If there is no influence from the Doppler effect, the delay time of the measurement light relative to the reference light corresponds to the distance d to the measurement object 10, and the frequency of the beat signal generated by the beat signal generation section 150 is _F0 . FIG. 10 shows an example in which an error ΔF occurs due to the Doppler effect, and the positive frequency F ₁ of the positive beat signal generated by the beat signal generator 150 during the period from time t _m to t _n+1 is F ₀ −ΔF. becomes.

Similarly, the reference light that reached the beat signal generator 150 during the period from time t _n+1 to t _n+2 is the laser light that was output from the laser device 110 during the period when the frequency shift direction of the optical SSB modulator 30 was set to the negative side. is part of. Further, the measurement light branched during the period when the frequency shift direction of the optical SSB modulator 30 is set to the negative side reaches the beat signal generation unit 150 during the period from time t _m+1 to t _m+2 delayed by a certain time. .

Here, since the error due to the Doppler effect shifts the frequency of the measurement light, the waveform of the measurement light is shifted in the vertical axis direction in the upper diagram of FIG. In this case, the delay time of the measurement light with respect to the reference light varies depending on the shift direction. For example, the negative frequency F ₂ of the negative beat signal generated by the beat signal generating section 150 during the period from time t _m+1 to t _n+2 is F ₀ +ΔF.

Therefore, as an example, when the frequency analysis unit 220 calculates the average value of the positive side frequency F ₁ and the negative side frequency F ₂ , the frequency analysis unit 220 ideally calculates the frequency F ₀ of the beat signal that would occur without the influence of the Doppler effect. You can get the value of Thereby, the frequency analysis section 220 can calculate the distance d between the optical head section 140 and the measurement target object 10 using equation (6). The display unit 170 displays the calculated value of the distance d.

As described above, the detection unit 160 detects the positive beat signal generated when the frequency shift direction of the optical SSB modulator 30 is set to the positive side, and the positive beat signal generated when the frequency shift direction of the optical SSB modulator 30 is set to the negative side. Based on the results of frequency analysis of the generated negative-side beat signals, it is possible to detect the difference in propagation distance between the reference light and the measurement light, which are less affected by the Doppler effect. Note that the measuring device 100 may measure the shape of the measurement object 10 by repeating the operation flow shown in FIG. 9 by changing the position at which the measurement object 10 is irradiated with the frequency modulated laser beam.

By using the laser device 110 having the optical SSB modulator 30, the measuring device 100 according to the present embodiment described above can reduce the influence of the Doppler effect and measure the distance d to the measurement target 10 with high accuracy. Here, from equation (6), the influence of the measurement accuracy of the signal frequency ν _B (m, d) of the beat signal on the measurement accuracy of the distance d becomes smaller as the values of ν _s and ν _c become larger. Although it is possible to increase the value of ν _c by shortening the resonator length of the laser device 110, there are limits to the length of the amplification medium 114 necessary for laser oscillation, the length necessary for handling the fiber, etc. , the value of ν _c cannot be easily increased.

When the AOFS 20 is used as the frequency shifter 112, the maximum frequency shift amount due to the optical crystal 21 is about 1.5 GHz, and from the viewpoint of efficiency etc., the shift amount is usually about several hundred MHz. Further, since the ultrasonic transducer 22 has a resonant frequency, there is a limit to its use with variable frequency shift amount.

On the other hand, since the measuring device 100 according to the present embodiment uses the optical SSB modulator 30, it is possible to set a frequency shift amount ν _s that is several tens of times larger than that of the AOFS 20. Therefore, the measuring device 100 can improve the sensitivity of the beat frequency to changes in the distance d by several tens of times compared to the conventional method, and can measure the distance d to the measurement target 10 with high precision.

Further, the measuring device 100 according to the present embodiment can also easily determine the order m of the beat signal, for example, by being able to change the shift amount of the frequency of the frequency shift feedback laser. As described in the explanation of equation (6), the order m can be determined by detecting the change in the beat signal when the frequency shift amount ν _s of the laser device 110 is changed.

As described in Patent Document 1, etc., such a method for determining the order m detects a beat signal for each different frequency shift amount, generates the relational expression of equation (5), and makes it simultaneous. This method is known as a method for calculating the order m as a solution of simultaneous equations. The measuring device 100 may determine the order m using such a method by changing the shift amount of the frequency of the frequency-shifted feedback laser.

Note that if the order m changes in accordance with a change in the amount of frequency shift, the number of solutions to the simultaneous equations will increase, and the number of equations to be made simultaneously must be correspondingly increased. The value of the order m can be expressed as the quotient of the distance d divided by the absolute measurement range as shown in the following equation. Here, the absolute length measurement range is the range in which the distance d can be continuously measured without changing the value of the order m, and the distance that the measurement light travels back and forth within the reciprocal of the shift frequency _νs (i.e., c /2ν _s ). Also, floor() means truncation of the decimal part.

Here, by differentiating equation (7) with ν _s , the following equation is obtained.

From equation (8), it can be seen that the larger the value of the distance d, the larger the amount of change in the value of the order m with respect to the change in the frequency shift amount ν _s . This means that the greater the distance d from the laser device 110 to the measurement target 10, the more likely the order m will change due to the frequency shift. Further, as described above, when the frequency shift amount ν _s is increased, the distance measurement accuracy is improved, but the absolute length measurement range is narrowed, and the order m is likely to change. Therefore, in order to accurately measure the object 10 that is farther away, the value of m changes more and the simultaneous equations become more complicated, which may make it difficult for the measuring device 100 to measure the object 10. Ta.

Therefore, the measuring device 100 according to the present embodiment has a higher sensitivity and a more distant distance d by differentiating the frequency shift amount for determining the order m and the frequency shift amount for measuring the distance d. can be measured. The measurement operation of such a measurement device 100 will be explained next.

[Second example of operation flow of measuring device 100]
FIG. 11 shows a second example of the operation flow of the measuring device 100 according to this embodiment. In the operation flow of the second example, a part of the explanation of the operations already explained in the operation flow of the first example will be omitted.

First, in S1110, the control unit 50 sets the frequency shift amount ν _s of the optical SSB modulator 30 to the first frequency ν _s1 . The first frequency ν _s1 is a frequency shift amount for determining the order m, and is a smaller value than the frequency shift amount for measuring the distance d. The first frequency ν _s1 is desirably a low frequency for expanding the absolute length measurement range for measuring the distance d to several times or more. Further, as described later, it is more desirable that the first frequency ν _s1 be a frequency corresponding to the distance d. The first frequency ν _s1 is, for example, a frequency on the order of several tens of MHz to several GHz. Further, the control unit 50 may further set the frequency shift direction of the optical SSB modulator 30.

Next, in S1120, the control unit 50 controls the laser device 110 having the amplification medium 114 and the optical SSB modulator 30 in the laser resonator to output frequency modulated laser light of a plurality of modes. Then, the branching unit 120 branches part of the frequency modulated laser light outputted by the laser device 110 into a reference light, and branches at least part of the remaining part into a measurement light. The optical head section 140 irradiates the measurement object 10 with measurement light. The optical head section 140 then receives the reflected light from the measurement target object 10. The beat signal generator 150 mixes the reflected light and the reference light to generate an order determination beat signal.

Next, in S1130, the control unit 50 controls the detection unit 160 to cause the detection unit 160 to frequency-analyze the order determination beat signal. Then, the frequency analysis unit 220 calculates the order determination frequency _F3 at which the order determination beat signal is generated.

Next, in S1140, the frequency analysis unit 220 determines the order m using the frequency _F3 for order determination and the equation (5). The frequency analysis unit 220 determines the order m in the absolute length measurement range for measuring the distance d. The relationship between the frequency shift amount ν _s and the absolute length measurement range will be explained using FIG. 12.

FIG. 12 shows the concept of the relationship between the frequency shift amount ν _s and the absolute length measurement range of the laser device 110 according to the present embodiment. The horizontal axis in FIG. 12 indicates the distance d to the measurement target 10, and the vertical axis indicates the frequency of the beat signal. As explained in FIG. 6, the plurality of straight lines shown by solid lines in FIG. 12 are graphs showing the relationship between the frequency ν _B (m, d) of the beat signal and the distance d for each of the plurality m. That is, the plurality of straight lines shown by solid lines indicate the relationship between the distance d and the frequency ν _B (m, d) when the control unit 50 sets the frequency shift amount ν _s2 for measuring the distance d in the laser device 110. . The frequency shift amount ν _s2 in this case is defined as the second frequency. Further, the absolute length measurement range is c/2ν _s =c/2ν _s2 .

The straight line shown by the dotted line in FIG. 12 shows the relationship between the distance d and the frequency ν _B (m, d) when the frequency shift amount ν _s is set to the first frequency ν _s1 . FIG. 12 shows an example in which the first frequency ν _s1 is set to ν _s2 /5. This shows that the absolute length measurement range has been expanded to 5c/2ν _s2 . Therefore, when the frequency shift amount ν _s is set to the first frequency ν _s1 and an order determination beat signal with a frequency in the range from 0 to ν _c is detected, the order determination beat signal corresponding to the distance d without any change in the order m is detected. Frequency _F3 can be detected.

As an example, assume that the detection unit 160 detects the order determination beat signal, performs frequency analysis, and calculates the frequency _F3 . In this case, since the frequency of the order determination beat signal is within the absolute length measurement range, the distance d ₃ corresponding to the frequency F ₃ can be calculated. Then, based on the calculated distance _d3 , it is possible to determine the order m when the frequency shift amount is set to the second frequency _νs . For example, from the plurality of straight lines shown as solid lines in FIG. 12, it can be seen that the absolute length measurement range in which the distance _d3 can be measured is of order m=3, and that beat signals generated by straight lines of other orders are replicas. As described above, by setting the first frequency ν _s1 to a lower frequency, the absolute length measurement range can be expanded and the order m when measuring the distance d can be determined. Note that the first frequency ν _s1 is desirably set to a frequency such that the frequency corresponding to the distance d to the measurement target object 10 is included in the expanded absolute length measurement range.

Next, in S1150, the control unit 50 switches the frequency shift amount ν _s of the optical SSB modulator 30 to the second frequency ν _s2 , which is larger than the first frequency ν _s1 . As described above, the second frequency ν _s2 is the frequency shift amount for measuring the distance d. The branching unit 120 branches part of the frequency modulated laser light whose frequency shift amount has been changed as reference light, and branches at least part of the remaining part as measurement light. The optical head section 140 irradiates the measurement object 10 with measurement light. The optical head section 140 receives reflected light reflected from the measurement target object 10. The beat signal generator 150 then mixes the reflected light and the reference light to generate a distance measurement beat signal.

Next, in S1160, the control unit 50 switches the frequency shift amount of the optical SSB modulator 30, and then controls the detection unit 160 to perform frequency analysis of the distance measurement beat signal. Then, the frequency analysis unit 220 calculates the distance measurement frequency _F4 at which the distance measurement beat signal is generated.

Next, in S1170, the frequency analysis unit 220 detects the difference in propagation distance between the reference light and the measurement light based on the results of frequency analysis of the order determination beat signal and the distance measurement beat signal. For example, the frequency analysis unit 220 calculates the distance d by substituting the order m and the frequency F ₄ for distance measurement into equation (6). The display unit 170 displays the calculated value of the distance d.

As described above, the measuring device 100 can easily determine the order m by reducing the amount of frequency shift so that it has resolution sufficient to at least determine the order m. Then, the distance d is measured after improving measurement accuracy by switching the frequency shift amount to a higher frequency. Thereby, the measuring device 100 can measure a farther distance d with higher sensitivity.

Note that in the operational flow of FIG. 11, an example has been described in which the measuring device 100 detects the order determination beat signal and then detects the distance measurement beat signal, but the present invention is not limited to this. The measuring device 100 may detect the order determination beat signal after detecting the distance measurement beat signal. In this way, the control unit 50 sets the frequency shift amount of the optical SSB modulator 30 to either the first frequency ν _s1 or the second frequency ν _s2 which is larger than the first frequency ν _s1 , and then the detection unit 160 The beat signal may be frequency analyzed. Then, the detection unit 160 uses the order determination beat signal generated when the frequency shift amount of the optical SSB modulator 30 is set to the first frequency ν _s1 and the order determination beat signal generated when the frequency shift amount of the optical SSB modulator 30 is set to the second frequency ν _s2 . The difference in propagation distance between the reference light and the measurement light is detected based on the results of frequency analysis of the distance measurement beat signals generated in each case.

In the measuring device 100 according to the present embodiment, the operation of reducing the influence of the Doppler effect and the operation of determining the order m have been described separately, but the present invention is not limited thereto. The measurement device 100 may determine the order m while reducing the influence of the Doppler effect. The operation of such a measuring device 100 will be described next.

[Third example of operation flow of measuring device 100]
FIG. 13 shows a third example of the operation flow of the measuring device 100 according to this embodiment. In the operation flow of the third example, a part of the description of the operations already explained in the operation flow of the first example and the operation flow of the second example will be omitted.

First, in S1210, the control unit 50 sets the frequency shift amount of the optical SSB modulator 30 to the first frequency ν _s1 , and sets the frequency shift direction to either the positive side or the negative side. As an example, the control unit 50 sets the first frequency to ν _s /5 and the frequency shift direction to positive.

Next, in S1220, the control unit 50 controls the laser device 110 to output frequency modulated laser light of a plurality of modes. Then, the branching unit 120 branches part of the frequency modulated laser light outputted by the laser device 110 into a reference light, and branches at least part of the remaining part into a measurement light. The optical head section 140 irradiates the measurement object 10 with measurement light. The optical head section 140 then receives the reflected light from the measurement target object 10. The beat signal generator 150 mixes the reflected light and the reference light to generate an order determination beat signal.

Next, in S1230, the control unit 50 controls the detection unit 160 to cause the detection unit 160 to analyze the frequency of the order determination beat signal. Then, the frequency analysis unit 220 calculates the order determination frequency F ₀ at which the order determination beat signal is generated.

Next, in S1240, the frequency analysis unit 220 determines the order m using the frequency F ₀ for order determination and the equation (5). At this stage, the frequency _F1 calculated based on the order determination beat signal may include an error due to the influence of the Doppler effect. However, the error caused by the Doppler effect is so small that it hardly affects the determination of the order m, and is not a problem here.

Next, in S1250, the control unit 50 switches the frequency shift amount of the optical SSB modulator 30 to a second frequency that is larger than the first frequency, and generates a positive beat signal. Here, the control unit 50 does not change the frequency shift direction of the optical SSB modulator 30.

Next, in S1260, the control unit 50 controls the detection unit 160 to frequency-analyze the positive beat signal. Then, the frequency analysis unit 220 uses the frequency analysis results to calculate the positive frequency _F1 at which the positive beat signal is generated. Even at this stage, the positive side frequency _F1 calculated based on the positive side beat signal may include an error due to the influence of the Doppler effect.

Next, in S1270, the control unit 50 changes the frequency shift direction to the opposite direction while keeping the frequency shift amount of the optical SSB modulator 30 at the second frequency, thereby generating a negative beat signal. For example, the control unit 50 switches the frequency shift direction of the optical SSB modulator 30 from the positive side to the negative side. The branching unit 120 branches a portion of the frequency modulated laser beam whose frequency shift direction has been switched as reference light, and branches at least a portion of the remaining portion as measurement light. The optical head section 140 irradiates the measurement object 10 with measurement light. The optical head section 140 receives reflected light reflected from the measurement target object 10. Then, the beat signal generating section 150 mixes the reflected light and the reference light to generate a negative beat signal.

Next, in S1280, the control unit 50 controls the detection unit 160 to frequency-analyze the negative beat signal. Then, the frequency analysis unit 220 uses the frequency analysis results to calculate the negative frequency _F2 at which the negative beat signal is generated. Even at this stage, the negative side frequency _F2 calculated based on the negative side beat signal may include an error due to the influence of the Doppler effect. In this case, the error included in the negative frequency F ₂ has substantially the same value and the opposite sign compared to the error included in the positive frequency F ₁ .

Next, in S1290, the frequency analysis unit 220 detects the difference in propagation distance between the reference light and the measurement light based on the results of frequency analysis of the order determination beat signal, the positive beat signal, and the negative beat signal. For example, the frequency analysis unit 220 calculates the average value of the positive frequency F ₁ and the negative frequency F ₂ . Thereby, the frequency analysis section 220 calculates the distance d between the optical head section 140 and the measurement target object 10 by substituting the average value and the order m into equation (6). The display unit 170 displays the calculated value of the distance d.

As described above, the measuring device 100 according to the present embodiment includes the laser device 110 that can easily change the frequency shift amount and frequency shift direction, so it can determine the order m while reducing the influence of the Doppler effect, The distance d to the measurement target 10 can be measured with high accuracy.

[Modified example of control unit 50]
In the measurement device 100 according to the present embodiment described above, the laser device 110 includes the optical SSB modulator 30, and by switching the voltage supplied to the optical SSB modulator 30, the frequency shift direction of the optical SSB modulator 30 is switched. Although an example in which this is possible has been described, the present invention is not limited to this. For example, the measuring device 100 may be able to switch the frequency shift direction of the optical SSB modulator 30 by inverting the phase of the RF signal that drives the optical SSB modulator 30. The optical SSB modulator 30 and control section 50 of such a measuring device 100 will be explained next.

FIG. 14 shows a modification of the control section 50 according to the present embodiment together with the optical SSB modulator 30. In the optical SSB modulator 30 and the control unit 50 of the modified example shown in FIG. 14, the same reference numerals are given to the operations that are substantially the same as those of the optical SSB modulator 30 and the control unit 50 according to the present embodiment shown in FIG. and omit the explanation. The control unit 50 of the modification further includes a phase difference generation unit 310 and a switching unit 320.

In the control unit 50 of the modified example, the RF signal generation unit 54 generates a plurality of RF signals. The RF signal generation unit 54 generates, for example, two RF signals that have substantially the same phase and frequency. The RF signal generation unit 54 may, for example, branch one RF signal into two. The RF signal generation section 54 supplies the plurality of generated RF signals to the phase difference generation section 310.

The phase difference generation unit 310 outputs a plurality of RF signals having a predetermined phase difference by varying the phases of a plurality of input RF signals. For example, the phase difference generation unit 310 converts two input RF signals having the same phase into three RF signals having a phase difference of approximately 90 degrees and outputs them. For example, if the phase of the first RF signal among the three RF signals output by the phase difference generation section 310 is 0 degrees, the phase of the second RF signal differs from the first RF signal by approximately 90 degrees, The phase of the third RF signal differs by approximately 180 degrees from the first RF signal.

For example, the phase difference generation unit 310 changes the phase of one of the two input RF signals by 90 degrees. Further, the phase difference generation section 310 generates an RF signal having a phase different by 180 degrees from the other RF signal of the two input RF signals. In other words, the phase difference generation unit 310 generates one RF signal (as an example, the above-mentioned second RF signal) and two RF signals (as an example, three RF signals including the above-mentioned first and third RF signals) are generated.

Alternatively, the RF signal generation section 54 may generate three RF signals having substantially the same phase and frequency and supply them to the phase difference generation section 310. In this case, the phase difference generation unit 310 changes the phase of one of the three input RF signals by 90 degrees, and also changes the phase of one of the remaining two RF signals by 180 degrees. The degree may be different.

Then, the phase difference generation unit 310 supplies one RF signal among the three RF signals, the phase difference of which differs by 90 degrees from the other two RF signals, to the first RF electrode 38 or the second RF electrode 39. Then, the phase difference generation section 310 supplies the remaining two RF signals to the switching section 320. FIG. 14 shows an example in which the phase difference generation section 310 supplies the second RF signal to the first RF electrode 38 and supplies the first RF signal and the third RF signal to the switching section 320.

The switching unit 320 supplies one of the two input RF signals to the first RF electrode 38 and the second RF electrode 39 to which the phase difference generating unit 310 is not supplying the RF signal. . FIG. 14 shows an example in which one of the two RF signals input to the switching unit 320 is input to the second RF electrode 39.

As a result, the phase difference between the two RF signals supplied to the first RF electrode 38 and the second RF electrode 39 becomes +90 degrees or -90 degrees. Further, the two phase differences can be switched by the control section 50 controlling the switching section 320. In this way, by inverting the sign of the phase difference of the RF signals supplied to the first RF electrode 38 and the second RF electrode 39 by the control unit 50, the frequency shift direction of the optical SSB modulator 30 can be switched. That is, the control unit 50 can switch the frequency shift direction of the optical SSB modulator 30 by switching the phase of the RF signal supplied to the optical SSB modulator 30.

As described above, switching the RF signal supplied to the optical SSB modulator 30 can be realized with a simple circuit configuration such as a switch. In this way, the control unit 50 of the modified example can have a simple circuit configuration, and therefore can switch the frequency shift direction stably and quickly at low cost.

Although the present invention has been described above using the embodiments, the technical scope of the present invention is not limited to the scope described in the above embodiments, and various modifications and changes can be made within the scope of the gist. be. For example, all or part of the device can be functionally or physically distributed and integrated into arbitrary units. In addition, new embodiments created by arbitrary combinations of multiple embodiments are also included in the embodiments of the present invention. The effects of the new embodiment resulting from the combination have the effects of the original embodiment.

10 Measurement object 20 AOFS
21 Optical crystal 22 Ultrasonic transducer 23 Ultrasonic absorber 24 Oscillator 30 Optical SSB modulator 31 Substrate 32 Main Mach-Zehnder waveguide 33 First sub-Mach-Zehnder waveguide 34 Second sub-Mach-Zehnder waveguide 35 Main DC electrode 36 First sub-DC electrode 37 Second sub-DC electrode 38 First RF electrode 39 Second RF electrode 41 First arm waveguide 42 Second arm waveguide 43 First sub-arm waveguide 44 Second sub-arm waveguide 45 Third sub-arm waveguide 46 Fourth sub-arm waveguide 50 Control section 52 DC voltage generation section 54 RF signal generation section 100 Measuring device 110 Laser device 112 Frequency shifter 114 Amplification medium 116 WDM coupler 117 Pump light source 118 Output coupler 120 Branch section 130 Optical circulator 140 Optical head section 150 Beat signal generation section 152 Optical 90 degree hybrid 154 Photoelectric conversion section 160 Detection section 162 First filter section 164 Second filter section 170 Display section 202 First AD conversion section 204 Second AD conversion section 210 Clock signal supply section 220 Frequency analysis section 310 Phase difference generation section 320 Switching Department

Claims (15)

A laser device that outputs frequency modulated laser light in multiple modes,
an amplification medium that amplifies input light;
an optical SSB modulator that shifts the frequency of the light amplified by the amplification medium;
A control unit that controls the optical SSB modulator to frequency shift the frequency of light input to the optical SSB modulator. The laser device according to claim 1, wherein the control section sets a frequency shift amount of the optical SSB modulator. The optical SSB modulator includes:
A substrate and
a main Mach-Zehnder waveguide provided on the substrate and having a first arm waveguide and a second arm waveguide;
a first sub-Mach-Zehnder waveguide provided in the first arm waveguide;
a second sub-Mach-Zehnder waveguide provided in the second arm waveguide;
The control unit applies a DC voltage of a predetermined value to electrodes provided on the substrate corresponding to the main Mach-Zehnder waveguide, the first sub-Mach-Zehnder waveguide, and the second sub-Mach-Zehnder waveguide. The laser device according to claim 2, wherein the laser device supplies an RF signal and changes the frequency to set the frequency shift amount. The laser device according to any one of claims 1 to 3, wherein the control unit sets a frequency shift direction of the optical SSB modulator. The optical SSB modulator includes:
A substrate and
a main Mach-Zehnder waveguide provided on the substrate and having a first arm waveguide and a second arm waveguide;
a first sub-Mach-Zehnder waveguide provided in the first arm waveguide;
a second sub-Mach-Zehnder waveguide provided in the second arm waveguide;
The control unit applies a DC voltage of a predetermined value to electrodes provided on the substrate corresponding to the main Mach-Zehnder waveguide, the first sub-Mach-Zehnder waveguide, and the second sub-Mach-Zehnder waveguide. 5. The laser device according to claim 4, wherein the frequency shift direction is switched by supplying the frequency shift direction. The optical SSB modulator includes:
A substrate and
a main Mach-Zehnder waveguide provided on the substrate and having a first arm waveguide and a second arm waveguide;
a first sub-Mach-Zehnder waveguide provided in the first arm waveguide;
a second sub-Mach-Zehnder waveguide provided in the second arm waveguide;
The control unit inverts the phase of an RF signal supplied to electrodes provided on the substrate corresponding to the main Mach-Zehnder waveguide, the first sub-Mach-Zehnder waveguide, and the second sub-Mach-Zehnder waveguide. 5. The laser device according to claim 4, wherein the frequency shift direction is switched. The laser device according to claim 1;
a branching section that branches part of the frequency modulated laser light outputted by the laser device as reference light and branches at least part of the remaining part as measurement light;
a beat signal generation unit that generates a beat signal by mixing the reference light with the reflected light that is reflected by irradiating the measurement light onto the measurement target;
A measuring device, comprising: a detection unit that frequency-analyzes the beat signal and detects a difference in propagation distance between the reference light and the measurement light. The laser device according to claim 2 or 3;
a branching section that branches part of the frequency modulated laser light outputted by the laser device as reference light and branches at least part of the remaining part as measurement light;
a beat signal generation unit that generates a beat signal by mixing the reference light with the reflected light that is reflected by irradiating the measurement light onto the measurement target;
A measuring device, comprising: a detection unit that frequency-analyzes the beat signal and detects a difference in propagation distance between the reference light and the measurement light. The laser device according to any one of claims 4 to 6,
a branching section that branches part of the frequency modulated laser light outputted by the laser device as reference light and branches at least part of the remaining part as measurement light;
a beat signal generation unit that generates a beat signal by mixing the reference light with the reflected light that is reflected by irradiating the measurement light onto the measurement target;
A measuring device, comprising: a detection unit that frequency-analyzes the beat signal and detects a difference in propagation distance between the reference light and the measurement light. The control unit sets the frequency shift amount of the optical SSB modulator to either a first frequency or a second frequency larger than the first frequency, and then causes the detection unit to frequency-analyze the beat signal. let me,
The detection unit detects an order determination beat signal generated when the frequency shift amount of the optical SSB modulator is set to the first frequency and when the frequency shift amount of the optical SSB modulator is set to the second frequency. detecting a difference in propagation distance between the reference light and the measurement light based on the results of frequency analysis of the distance measurement beat signals generated in the
The measuring device according to claim 8. The control unit switches the frequency shift direction of the optical SSB modulator to either a positive side or a negative side, and then causes the detection unit to frequency analyze the beat signal,
The detection unit detects a positive beat signal generated when the frequency shift direction of the optical SSB modulator is set to the positive side, and a negative beat signal generated when the frequency shift direction of the optical SSB modulator is set to the negative side. detecting a difference in propagation distance between the reference light and the measurement light based on the results of frequency analysis of the side beat signals;
The measuring device according to claim 9. The measuring device according to any one of claims 7 to 11, wherein the beat signal generator performs orthogonal detection of the reflected light and the reference light. setting the frequency shift direction of the optical SSB modulator to either the positive side or the negative side;
outputting a plurality of modes of frequency modulated laser light from a laser device having an amplification medium and the optical SSB modulator in a laser resonator;
Part of the frequency modulated laser light output by the laser device is used as reference light, and at least part of the remaining part is branched as measurement light;
irradiating the measurement light onto the measurement target and mixing the reflected light and the reference light to generate a positive beat signal;
switching the frequency shift direction of the optical SSB modulator to the opposite direction to generate a negative beat signal;
A measuring method comprising: detecting a difference in propagation distance between the reference light and the measurement light based on a result of frequency analysis of the positive beat signal and the negative beat signal. setting the frequency shift amount of the optical SSB modulator to a first frequency;
outputting a plurality of modes of frequency modulated laser light from a laser device having an amplification medium and the optical SSB modulator in a laser resonator;
Part of the frequency modulated laser light output by the laser device is used as reference light, and at least part of the remaining part is branched as measurement light;
generating an order determination beat signal by mixing the reference light with the reflected light that is reflected by irradiating the measurement light onto the measurement target;
switching the frequency shift amount of the optical SSB modulator to a second frequency larger than the first frequency to generate a distance measurement beat signal;
A measuring method comprising: detecting a difference in propagation distance between the reference light and the measurement light based on a result of frequency analysis of the order determination beat signal and the distance measurement beat signal. setting the frequency shift amount of the optical SSB modulator to a first frequency, and setting the frequency shift direction to either the positive side or the negative side;
outputting a plurality of modes of frequency modulated laser light from a laser device having an amplification medium and the optical SSB modulator in a laser resonator;
Part of the frequency modulated laser light output by the laser device is used as reference light, and at least part of the remaining part is branched as measurement light;
generating an order determination beat signal by mixing the reference light with the reflected light that is reflected by irradiating the measurement light onto the measurement target;
switching the frequency shift amount of the optical SSB modulator to a second frequency larger than the first frequency to generate a positive beat signal;
generating a negative beat signal by switching the frequency shift direction of the optical SSB modulator to the opposite direction while keeping the frequency shift amount at the second frequency;
A measuring method comprising: detecting a difference in propagation distance between the reference light and the measurement light based on the order determination beat signal, the positive beat signal, and the negative beat signal.

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